Recently, electronic apparatuses have been downsized. There is a strong demand to inexpensive circuit board, to which a semiconductor chip such as a large-scale integrated circuit (LSI) can be mounted, for not only industrial use but also consumer use. For downsizing the circuit board by increasing mounting density, it is important that many finer wiring layers can be produced easily at high yield and high reliability. When a through hole for electrically interconnecting the wiring layers formed on both surfaces of an insulating substrate is filled and used for mounting components, the circuit board can be downsized by increasing mounting density.
In a conventional circuit board, a glass epoxy substrate formed by impregnating glass woven fabric with epoxy resin is used as the insulating substrate. A roughed copper foil is thermally pressed onto the insulating substrate, the insulating substrate is masked in a desired pattern with photosensitive resin, and then a wiring pattern is formed by photo etching. In the photo-etching, etching solution containing iron chloride or copper chloride is sprayed on the insulating substrate.
In the photo etching, a micro pattern is hardly formed because side etching occurs to make the etched pattern smaller than the photosensitive resin pattern. Therefore, the copper foil is thinned for reducing the influence of the side etching, or a copper foil having low roughness is employed for narrowing a rough part embedded into the resin in the copper foil.
When the copper foil is thinned, however, electrical resistance of the wiring increases, and time constant of an electrical characteristic increases. In this case, a circuit board having such a copper foil cannot be employed as a circuit board for high frequency use.
When a copper foil having a shallowly roughed part, namely low roughness, is embedded into the resin so that the etching period of a part having a roughed uneven layer is shortened to reduce the side etching, a contact area between the substrate and the copper foil is decreased or an anchor effect is reduced. Peeling strength therefore decreases.
When the copper foil having the low roughness is employed in a method in which the through hole is not plated but filled with conductive material, not only the peeling strength decrease but also the connecting resistance increase due to the decrease of the contact area with the conductive material. Therefore, when the copper foil having the low roughness is employed, the electrical characteristic cannot be satisfied due to formation of the micro pattern.